Thick sections and non-uniform walls cause problems
Keep wall thickness as uniform as possible.
- Use gradual transitions between thick and thin sections.
- Wall thickness must suit both function and process.
- Wall thickness guide range is
0.75 mm to 3 mm for reinforced materials
0.5 mm to 5 mm for unreinforced materials
Un-even wall thickness tend to have bad injection result, both accuracy and strength.
Parts that might be made as solid shapes in traditional materials must be formed quite differently in plastics.
Moulded plastics do not lend themselves to solid forms. There are two principal reasons for this. First, plastics are processed
with heat but are poor conductors of heat. This means that thick sections take a very long time to cool and so are costly to make.
The problems posed by shrinkage are equally severe. During cooling, plastics undergo a volume reduction. In thick sections, this either
causes the surface of the part to cave in to form an unsightly sink mark
or produces an internal void. Furthermore, plastics materials are expensive; it is only high-speed production methods and net-shape forming that make mouldings viable. Thick sections waste material and are simply uneconomic.
So solid shapes that would do the job well in wood or metal must be transformed to a ‘shell’ form in plastics. This is done by hollowing out
or ‘coring’ thick parts so you are left with a component which regardless of complexity is composed essentially of relatively thin walls joined
by curves, angles, corners, ribs, steps and offsets. As far as possible, all these walls should be the same
thickness. It is not easy to generalise what the wall thickness should be. The wall plays a part both in design concept and embodiment. The wall
must be thick enough to do its job; it must be strong enough or stiff enough or cheap enough. But it must also be thin enough to cool quickly
and thick enough to allow efficient mould filling. If the material is inherently strong or stiff the wall can be thinner. As a general guide, wall
thicknesses for reinforced materials should be 0.75 mm to 3 mm, and those for unfilled materials should be 0.5 mm to 5 mm. Ideally, the entire
component should be a uniform thickness – the nominal wall thickness. In practice that is often not possible; there must be some variation in
thickness to accommodate functions or aesthetics. It is very important to keep this variation to a minimum. A plastics part with thickness
variations will experience differing rates of cooling and shrinkage. The result is likely to be a part that is warped and distorted, one in which
close tolerances become impossible to hold. Where variations in thickness are unavoidable, the transformation between the two should be
gradual not sudden so instead of a step, use a ramp or a curve to move from thick to thin.